HNPCC (Lynch syndrome) is one of the most common cancer predisposition syndromes, affecting as many as 1 in 200 individuals in the western world (Lynch et al., 1993). Affected individuals develop tumors of the colon, endometrium, ovary and other organs, often before 50 years of age. Although the familial nature of this syndrome was discovered nearly a century ago (Warthin et al., 1913), the role of heredity in its causation remained difficult to define (Lynch et al., 1966). Recently, however, linkage analysis in two large kindreds demonstrated association with polymorphic markers on chromosome 2 (Peltomaki et al., 1993a). Studies in other families suggested that neoplasia in a major fraction of HNPCC kindreds is linked to this same chromosome 2p locus (Aaltonen et al., 1993).
HNPCC is defined clinically by the occurrence of early-onset colon and other specific cancers in first degree relatives spanning at least two generations (Lynch et al., 1993). The predisposition is inherited in an autosomal dominant fashion. It was initially expected that the gene(s) responsible for HNPCC was a tumor suppressor gene, as other previously characterized cancer predisposition syndromes with this mode of inheritance are caused by suppressor gene mutations (reviewed in Knudson, 1993). But the analysis of tumors from HNPCC patients suggested a different mechanism. Most loci encoding tumor suppressor genes undergo somatic losses during tumorigenesis (Stanbridge, 1990). In contrast, both alleles of chromosome 2p loci were found to be retained in HNPCC tumors (Aaltonen et al., 1993). During this search for chromosome 2 losses, however, it was noted that HNPCC tumors exhibited somatic alterations of numerous microsatellite sequences.
Widespread, subtle alterations of the cancer cell genome were first detected in a subset of sporadic colorectal tumors using the arbitrarily-primed polymerase chain reaction (Peinado et al., 1992). These alterations were subsequently found to represent deletions of up to 4 nucleotides in genomic polyA tracts (Ionov et al., 1993). Other studies showed that a similar, distinctive subgroup of sporadic tumors had insertions or deletions in a variety of simple repeated sequences, particularly microsatellite sequences consisting of dinucleotide or trinucleotide repeats (Ionov et al., 1993; Thibodeau et al., 1993; Aaltonen et al., 1993). Interestingly, these sporadic tumors had certain features in common with those developing in HNPCC kindreds, such as a tendency to be located on the right side of the colon and to be near-diploid. These and other data suggested that HNPCC and a subset of sporadic tumors were associated with a heritable defect causing replication errors (RER) of microsatellites (Ionov et al., 1993; Aaltonen et al., 1993).
The mechanism underlying the postulated defect could not be determined from the study of tumor DNA, but studies in simpler organisms provided an intriguing possibility (Levinson and Gutman, 1987; Strand et al., 1993). This work showed that bacteria and yeast containing defective mismatch repair genes manifest instability of dinucleotide repeats. The disruption of genes primarily involved in DNA replication or recombination had no apparent effect on the fidelity of microsatellite replication (reviewed in Kunkel, 1993). These pivotal studies suggested that defective mismatch repair might be responsible for the microsatellite alterations in the tumors from HNPCC patients (Strand et al., 1993).
Thus there is a need in the art to identify the actual gene and protein responsible for hereditary non-polyposis colorectal cancer and the replication error phenotype found in both hereditary and sporadic tumors. Identification of the gene and protein would allow more widespread diagnostic screening for hereditary non-polyposis colorectal cancer than is currently possible. Identification of the involved gene and protein would also enable the rational screening of compounds for use in drug therapy of hereditary non-polyposis colorectal cancer, and would enable gene therapy for affected individuals.